The synthesis, structures, spectroscopic properties, electrochemical properties and reactivities of specific Fe/M/S clusters (M=Mo, W, V), Fe/S clusters and Mo/S/O complexes will be the specific aims of this proposal. The Fe/M/S clusters are expected to serve as synthetic analogs for the Fe/M/S centers in the nitrogenases and alternate nitrogenases that contain vanadium or iron in place of molybdenum. One type of Fe/S clusters will be synthesized as models for the active site in sulfite and nitrite reductase. Other synthetic Fe/S clusters will be the "mixed" terminal ligand Fe6S6 "prismanes" expected to serve as models for the new ferredoxins that contain the Fe6S6 metal cores. A great number of Mo containing enzymes (oxotransferases) catalyze the oxidation of organic substrates by oxo-transfer reactions that involve specific Mo[VI]=O units. These units are part of a common Mo-cofactor that invariably is found coordinated to a pterin-bound dithiolene ligand. The function of this "non-innocent" ligand at present is not understood and will be subject to our studies. Electron transfer in biological systems and the oxidation and reduction of various substrates in living organisms are of fundamental importance to life sustaining processes. Multi-electron transfer processes are less frequent in nature but are of equally great importance. Included among these are: The 4e- oxidation of 2H20 to 02 in photosynthesis, the 6e- reductions of N2 to ammonia in nitrogenase, and the 6e- reduction of sulfite to sulfide and of nitrite to ammonia in sulfite and nitrite reductases respectively. A general feature that appears to be associated with the sites involved in biological, multielectron redox processes, is a complex supermolecular structure that consists of bridged, redox active subunits. The factors that affect multielectron redox processes within such superstructures are not understood, and appropriate synthetic clusters that may be used as models are not readily available. This proposal is concerned with the synthesis and study of models for nitrogenase and sulfite reductase and of super-molecular assemblies that contain redox-active subunits. The effects of the bridging ligands between subunits, on the redox potentials and the competency of the assemblies to affect specific oxidations or reductions of substrate molecules, will be evaluated.